The present invention relates to a liquid crystal display device that can reflect light in both the visible and infrared ranges of the electromagnetic spectrum, and to a method of fabricating such a device.
Cholesteric displays are bistable in the absence of a field, the two stable textures being the reflective planar texture and the weakly scattering focal conic texture. In the planar texture, the helical axes of the cholesteric liquid crystal molecules are substantially parallel to the substrates between which the liquid crystal is disposed. In the focal conic state the helical axes of the liquid crystal molecules are generally randomly oriented. By adjusting the concentration of chiral dopants in the cholesteric material, the pitch length of the molecules and thus, the wavelength of radiation that they will reflect, can be adjusted. Cholesteric materials that reflect infrared radiation have been used for purposes of scientific study. Commercial displays are fabricated from cholesteric materials that reflect visible light.
Liquid crystal displays are useful as instrumentation in vehicles. For example, commercial airlines employ LCD instrumentation in the cockpits. Vehicles such as for military use, may use LED or LCD instrumentation. In military vehicles used to conduct stealth night operations, such as army helicopters, pilots wear night vision detectors or goggles that enable them to view objects in the air and on the ground without using visible light. The night vision goggles enable the wearer to view infrared radiation, such as the heat from the motor of an automobile. The night vision goggles may also utilize the ambient infrared light from the night sky to view objects that do not emit infrared radiation. The night vision goggles are worn spaced from the eyes of the pilot so that the LED instrumentation panels can be read when the wearer looks down, without looking through the goggles. Use of current night vision goggles limits the depth perception of the wearer. In addition, visible light may saturate the night vision goggles and render them ineffective. The goggles thus may filter out certain wavelengths of visible light.
The present invention is directed to a liquid crystal display including a single chiral nematic liquid crystal material or at least two chiral nematic liquid crystal materials, which can reflect light across a particular range of wavelengths. One aspect of the invention is directed to a display that reflects in both the visible and infrared ranges of the electromagnetic spectrum at an intensity that is observable to the human eye. The radiation in the infrared spectrum is observed using a device suitable for detecting infrared radiation, such as night vision goggles. Another aspect of the invention is the cell wall configurations of the displays. The invention may employ a single cell using one chiral nematic liquid crystal that reflects in the visible and in the infrared ranges or at least two liquid crystal cells each having a different chiral nematic liquid crystal disposed in each. When two or more cells are used, the cells may be stacked on top of one another. The chiral nematic liquid crystal composition may be tailored to have a certain peak intensity and bandwidth according to the invention. Although the preferred operation of the inventive display utilizes light reflecting from the liquid crystal, it would be appreciated by those skilled in the art in view of this disclosure that the display may be used in a transmissive mode using backlighting.
In general, the present invention is directed to a liquid crystal display device comprising cell wall structure and a chiral nematic liquid crystal material. The cell wall structure and the liquid crystal cooperate to form focal conic and twisted planar textures that are stable in the absence of a field. A device applies an electric field to the liquid crystal for transforming at least a portion of the material to at least one of the focal conic and twisted planar textures. The liquid crystal material reflects radiation having a wavelength in both the visible and the infrared ranges of the electromagnetic spectrum at intensity that is sufficient for viewing by an observer. In particular, the liquid crystal has a positive dielectric anisotropy. At least about 20% of the radiation incident on the material is preferably reflected from the material. The liquid crystal material may have an optical anisotropy of at least about 0.10.
In one embodiment of the invention, the display device employs a single liquid crystal material that is disposed in one region and yet reflects both visible and ago infrared radiation. This is accomplished by selecting the peak reflection wavelength of the radiation and by broadening the bandwidth or the range of wavelengths in which the radiation is reflected.
Another embodiment of the invention utilizes two regions, a liquid crystal material being disposed in each region. The cell wall structure forms a first region in which a first chiral nematic liquid crystal material is disposed and a second region in which a second chiral nematic liquid crystal material is disposed. The first liquid crystal reflects radiation having a wavelength in the visible range and the second liquid crystal material reflects radiation having a wavelength in the infrared range.
The particular cell wall structure that is used to form the two regions may be a stacked display employing three, four or more substrates. The liquid crystal material is disposed between opposing substrates. In one aspect using four substrates the first region is disposed between first and second substrates and the second region is disposed between third and fourth substrates. The first and second regions are arranged in series with respect to one another in the direction toward the observer. In this regard, the first liquid crystal reflecting visible light is disposed downstream of the liquid crystal reflecting infrared radiation in the direction toward the observer. In the case of the three substrate stacked display, the first region is disposed between first and second substrates and the second region is disposed between the second substrate and a third substrate.
The spacing between substrates in the single cell display ranges from about 4 to about 10 microns. The spacing between the substrates in the stacked cell a display is at least about 4 microns.
The three cell display employs a photolithography method of the present invention to form a substrate that employs patterned electrodes on both sides. This substrate can be used in any stacked display. This method of the invention includes applying radiation in the ultraviolet region of the electromagnetic spectrum through a mask. The radiation is reflected through a substrate, each opposing surface of the substrate containing a layer of photoresist material over a conductive layer disposed on the surface. The photoresist layer is exposed to the UV radiation on both sides of the substrate. Exposed photoresist material and underlying electrode material are removed from the substrate to form an electrode pattern on both surfaces of the substrate.
In particular, the ultraviolet radiation is applied at a level effective to compensate for the transmission loss of the photoresist, the electrode and the substrate. The ultraviolet radiation is applied at a level that is at least two times the level of ultraviolet radiation that is normally used to expose photoresist on a substrate.
A preferred embodiment of the present invention is directed to instrumentation of the type that is used by personnel employing a night vision detector such as goggles. The instrumentation reflects light having a wavelength in the visible region of the electromagnetic spectrum. This embodiment of the present invention has military applications, such as use in instruments in the cockpit of army helicopters. The present invention includes a liquid crystal display device comprising cell wall structure and a chiral nematic liquid crystal material. The cell wall structure and the liquid crystal cooperate to form focal conic and twisted planar textures that are stable in the absence of a field. A device applies an electric field to the liquid crystal for transforming at least a portion of the material to at least one of the focal conic and twisted planar textures. The liquid crystal material can reflect radiation having a wavelength in the visible and infrared regions of the spectrum at an intensity sufficient for viewing by the personnel.
Another aspect of the present invention is a multicolor stacked cell display that reflects infrared and visible radiation. The display device comprises cell wall structure and a chiral nematic liquid crystal material. The cell wall structure and the liquid crystal cooperate to form focal conic and twisted planar textures that are stable in the absence of a field. The cell wall structure forms first, second and third regions in which first, second and third chiral nematic liquid crystal materials are disposed, respectively. A device applies an electric field to at least one of the first, second and third liquid crystal materials for transforming at least a portion of these materials to at least one of the focal conic and twisted planar textures. The first and second liquid crystal materials have a pitch length effective to reflect radiation in the visible range of the electromagnetic spectrum and the third liquid crystal has a pitch length effective to reflect radiation in the infrared range of the spectrum. The visible and infrared radiation has an intensity sufficient for viewing by an observer.
Particular features of the color display are that the first liquid crystal may have a pitch length effective to reflect light of a first color and the second liquid crystal material may have a pitch length effective to reflect light of a second color. The display may include at least one other region in which a liquid crystal material that can reflect light in the visible range. For example, three visible cells may be used, resulting in a full color display.
In the stacked color display, when using substrates having patterned electrodes on only one side, the first region is disposed between first and second substrates, the second region is disposed between third and fourth substrates and the third region is disposed between fifth and sixth substrates. Alternatively, when using a substrate with electrodes patterned on both sides, the first region is disposed between first and second substrates, the second region is disposed between the second substrate and a third substrate and the third region is disposed between the third substrate and a fourth substrate. The first and the second regions are disposed downstream of the third region with respect to the direction from the display toward the observer. The invention may also include at least one colored material layer or a black layer transparent to infrared radiation. The colored material is disposed at the back substrate of a visible cell that is adjacent the infrared cell. The infrared transparent black layer may be disposed at the back of a visible cell. Also, a black layer may be adjacent the rearmost substrate of the infrared cell.
A method of making a display that can reflect infrared and visible radiation according to the invention includes adjusting the pitch length of a chiral nematic liquid crystal material so that the material reflects radiation having a wavelength in the visible and in the infrared ranges of the electromagnetic spectrum. Opposing substrates are spaced apart at a distance effective to provide the visible and infrared radiation with an intensity sufficient for viewing by an observer. The material is filled between the substrates such that the cell wall structure cooperates with the liquid crystal to form focal conic and twisted planar textures that are stable in the absence of a field. Also, connected is device for applying an electric field to the liquid crystal for transforming at least a portion of the material to at least one of the focal conic and twisted planar textures. The bandwidth of reflectance from the display may be broadened by using liquid crystal material having an optical anisotropy of at least about 0.10.
The present invention offers numerous features and advantages that have heretofore not been possible. A display reflecting both visible and infrared radiation enables use during the night and day, without compromising the electrooptical characteristics of the display. Moreover, the stacked cell feature of the invention enables ease of manufacture and modification for various applications. For example, variations in color and contrast may be attained utilizing colored or black layers on one or more of the substrates. Both cells of any stacked display, by tailoring the chiral nematic liquid crystal material in each cell, may be operated utilizing the same waveforms and driving voltages.
The photolithography method of the invention reduces the scattering of the stacked display. Also, no index matching material is needed between substrates. The method exposes the photoresist on both sides of the substrate using a single exposure step. Without this step, separate photolithography and wet chemical etching would have to be performed on each side of the substrate to pattern the electrodes. Also, for high resolution displays greater than 100 dots per inch, the electrode patterns must be registered to within 10 microns to avoid parallax problems. The double exposure technique cuts the photolithography and etching steps in half while automatically aligning the electrode patterns, since only one UV exposure is used to expose photoresist coated on both sides of the substrate.
The display of the invention may employ frontlighting and can utilize ambient visible or infrared radiation. Those skilled in the art would also appreciate that the invention may be modified to be suitable for backlighting. The display may be fabricated to include a device for directing either visible or infrared radiation onto the display. Alternatively, infrared radiation may be reflected from the night vision goggles toward the infrared reflecting display. In the case of military vehicles such as helicopters, the apparatuses surrounding the cockpit, for example, may provide ambient infrared radiation sufficient to illuminate the display. So, too, may visible light from instrumentation in the cockpit be sufficient to illuminate the visible display.
The present invention would be useful in any application in which it is desirable to have a display reflecting in the infrared and visible ranges. The invention may be suitable for use in instrumentation in helicopter or airplane cockpits, such as those that include numerical displays. Other applications include a display that can reflect infrared and visible light for use in a global positioning system that enables the user to determine his location based upon satellite information. Such a display could be used by foot soldiers employing night vision goggles who can read the display using only infrared radiation. In instances in which night vision goggles are used, since the wearer can view the infrared reflecting display through the goggles, the goggles may be worn closer to the face. This may improve viewing through the goggles.
Many additional features, advantages and a fuller understanding of the invention will be had from the accompanying drawings and the detailed description that follows.